Evaluation of the nutritional value of sorghum grain for rainbow trout, Oncorhynchus mykiss and hybrid striped bass, Morone chrysops × Morone saxatilis

Abstract

Aquaculture feeds with optimum digestible starch levels can provide benefits but only through the continued identification and characterization of the available nutrient content of novel or lesser utilized starch sources for a larger variety of aquatic species. To address this literature gap, in vivo digestibility trials were conducted in rainbow trout and hybrid striped bass to determine the available nutrient content of commercially sourced U.S. grain sorghum hybrids. Based on digestibility data, a regression design was employed to test the replacement of wheat flour with U.S. grain sorghum in practical-type diets for rainbow trout and hybrid striped bass at four inclusion levels (0%, 5%, 10%, and 20%). All diets were formulated to contain 40% digestible protein and 18% crude lipid, and balanced to available lysine, methionine, threonine, and phosphorus to targets of 3.82, 1.30, 2.14, and 0.6, respectively, prior to cooking extrusion. For the growth trials, 10 rainbow trout (59.1 ± 0.07 g, initial weight) or 10 hybrid striped bass (27.1 ± 0.1 g) were randomly stocked into triplicate replicate tanks per diet (300 or 500 L, respectively) and fed for eight or nine weeks, respectively to assess effects on growth performance. No significant negative effects of U.S. grain sorghum inclusion on hybrid striped bass final fish weight, growth rate expressed as a percent increase, feed conversion ratio, feed intake, body condition indices, or whole-body proximate composition were observed. The effects of 20% red grain sorghum inclusion on rainbow trout final fish weight were explained by the linear model: $\text{Final fish weigh}\left(g\right)=248-1.0\left(\text{Sorghum inclusion level}\right)$. The reduced growth observed in rainbow trout at the 20% inclusion level underscores the need for additional research to examine the potential beneficial effects of further sorghum processing and optimize feed extrusion parameters when U.S. grain sorghum is used in place of wheat flour.